This invention relates generally to closures for air ducts and, more particularly, to closures of the type having a hingedly interconnected blade assembly movable from a folded to an unfolded position.
Fire damper closures are mounted in the ducts or passageways in buildings and are automatically or manually operable to close off the duct or passage in the event of fire or smoke conditions. Fire damper closures conventionally comprise a plurality of blades which are hingedly interconnected into an assembly which is normally maintained in a folded condition and then unfolded in response to detection of smoke or excessive heat.
It has been found desirable in conventional fire damper closures to construct the blade assembly in a manner so that the blades rotate relative to each other less than 180.degree. upon closing of the assembly to the unfolded position. Such a construction offers advantages in strength and also facilitates resetting of the blades to the folded position. It has been recognized, however, that the limited rotation of the blades may result in a "spring action" which does not assure positive closing of the assembly. This problem has previously been overcome with a blade assembly of the type disclosed in U.S. Pat. No. 3,907,020 wherein the blades are interconnected in a manner which provides a positive stop to limit the degree of opening. The blade assembly disclosed in that patent also employs negator type coiled springs secured to the leading blade to assist with closing of the blade assembly.
When closure of the above described blade assembly is unaided by gravitational forces, the application of a biasing force to the leading or endmost blade of a foldable blade assembly is often insufficient to ensure complete closing of the blade assembly under high air flow conditions. It has been determined by the present inventor that at least two factors contribute to impede full closure of the assembly. First, the blades in the leading portion of the assembly tend to unfold to their maximum permitted orientation in response to air flow and pressure directed against those blades while the remaining blades remain in the folded position. As a result, the folded portion of the blade assembly is not drawn into the air flow where the pressure applied to those blades would otherwise contribute to unfolding and closing of the blade assembly. Second, the rate at which the blades unfold is impeded by the air flow and pressure which forces the unfolded blades against the damper frame. The additional frictional force of the blades moving along the damper frame then prevents the spring biasing force from completely closing the blade assembly. Despite attempts to increase, even double, the spring force exerted on the leading blade, complete closure of the blade assembly could not be reliably achieved with conventional spring assisted closure constructions.